Process for distilling sea water



Feb. 22, 1966 J. E. POTTHARST, JR 3,236,748

PROCESS FOR DISTILLING SEA WATER Original Filed April 21, 1960 2Sheets-Sheet 1 WATER sEAL FIG.

SEPARATO R TO WASTE CON DENSER 1 DISTILLATE T0 WASTE FEED IN DISTILLEDWATER OUT P m T N R A H T T O DI E N H O J A T TORNE YS.

Feb. 22, 1966 J. E. POTTHARST, JR 3,236,748

PROCESS FOR DISTILLING SEA WATER 2 Sheets-Sheet 2 Original Filed April21, 1960 4 ,II, 7 I

U OJ 5 7 6 5 4 2 2 Igorv ic b uc lzoi ti o I50 I50 |Eo I I T5o 2 0 20220 2 0 F TEMPERATURE OF HEAT EXCHANGE SURFACE (SEA WATER SIDE) Fl G.III

PRECIPITATION 0:41 ZOFGEPZMQZOQ F TEMPERATURE OF HEAT EXCHANGESURFACE(SEA WATER SIDE) F I G 4.. INVENTOR.

JOHN e. POTTHARST, JR.

JYQQWM his ATTORNEYS.

nited States Patent 3,236,748 PRUiIEiS FUR DESTKLILHNG SEA WATER John E.Pottharst, in, 861 Carondelet St, New Urieans, La. Continuation ofapplication Ser. No. 23,751, Apr. 21, 1960. This application May 21,1364, Ser. No. 377,160 13 Claims. ((31. 2113-11) This invention relatesto a process for distilling sea Water which eliminates the formation ofscale on the heat exchange surfaces of the distillation apparatus.

This is a continuation of the copending application of John E.Pottharst, Jr, Serial No. 23,751, filed April 21, 1960, which is acontinuation-in-par-t of Serial No. 726,362, filed April 4, 1958.

In recent years, particularly in view of the increased range of navalvessels, the increased activity in tideland oil drilling and the generalinterest in producing potable water from sea water, there has arisen agreat demand for a more eflicient method and apparatus for distillingsea water, with particular emphasis on eliminating the crystallizationand scaling of the metal heat exchange surfaces with which the sea watercomes in contact. It is no exaggeration to say that this is one of themost pressing needs of these times. Many proposals have been advanced tominimize crystallization and scaling of sea water, and some of theseproposals have helped in the control thereof. The fact remains, however,that even today with all of the efforts that have been made to minimizescaling, it is still necessary to shut down sea water distillation unitsat intervals of approximately every two weeks or so in order to subjectthe equipment to an acid treatment for the purpose of removing scale andother debris which forms on and adheres to the heat exchange surfaces,thereby greatly reducing the efiiciency of distilling sea water.

The principal object of the present invention is to provide a practicaland eflicient process for distilling sea water which eliminates thetendency of the sea water to form scale on the heat exchange surfaces.

These objects have been achieved by the discovery of certain criticalfactors which determine the tendency and rate of formation of scale onthe heat exchange surfaces. The present invention, therefore,contemplates careful control over certain of the operating conditionsunder which the vaporization of sea water is carried out, namely,control of the temperature of the heat exchange surface against whichthe liquid to be vaporized comes into contact, control of the pressureat which the vaporization of the sea water is carried out, control ofthe concentration of the sea water to be distilled, particularly byregulation of the relationship between the rate at which theconcentrated sea Water is discharged as blowdown to the rate at whichfeed water is supplied, and control of the velocity of the sea Water tobe distilled relative to the heat exchange surface with which it comesinto contact.

For a complete understanding of the present invention, reference may bemade to the detailed description which follows, and to the accompanyingdrawings, in which:

FIGURE 1 is a schematic cross-sectional view of a distillation unitsuitable for carrying out the present in vention;

FIGURE 2 is a greatly enlarged view of a heat exchange surface whichforms part of the apparatus shown in FIGURE 1;

FIGURE 3 is a curve which shows the relationship between the Weight ofdeposition of scale on the surface with which the sea water comes intocontact and the temperature of the heat exchange surface at a specifiedconcentrat-ion of the sea water;

FIGURE 4 is a curve which shows the point at which ice scaling begins toform at different concentration ratios and at different temperatures ofthe evaporating film of sea water at zero velocity of the sea water.

To facilitate an understanding of the present process, the apparatusshown in FIGURE 1 of the drawings will be briefly described. Theapparatus shown in FIGURE 1 is the subject matter of the copendingapplication of John E. Pottharst, Jr, Serial No. 7,374, filed February8, 1960.

Referring to FIGURE 1, the sea water to be distilled is supplied by apump 10 through conduits 11 and 12 to a housing 13 of the distillationunit, generally designated A. Before being admitted to the distillationunit, the sea water is brought into out-of-contact heat-exchangerelationship with both the distillate manufactured by the unit A and theundistilled concentrated liquid or blowdown discharged as wastetherefrom. The housing 13 contains a vaporization chamber having a lowerliquid region and an upper vapor region. Before being admitted into thevaporization chamber, the sea water passes through a manually operatedvalve 15 and an automatically operated valve 16.

The lower liquid region of the vaporization chamber includes a liquidreservoir which is defined by a funnelsh-aped wall 18 surrounded by avoid 17 The lower end of the funnel-shaped wall 13 communicates with thecenter of a blade-carrying rotor or circulator 19 which is mounted atthe bottom of the vaporization chamber for rotation about a verticalaxis. The rotor is driven by suitable drive means (not shown) through ashaft 20 which extends through a seal 21 accommodated in the bottom wallof the housing 13.

The rotor 19 operates to maintain a continuous circulation of the seawater in a toroidal pattern of flow within the vaporization chamber.More specifically, the lower end of the funnel-shaped Wall 18 forms anaxial intake for the circulator 19 so that a downward flow of the liquidis maintained within the reservoir. An annular wall 23 spaced above thebottom wall of the housing 13 cooperates with the said bottom wall toform a radial discharge passage for the liquid circulated by the rotor.The wall 23 is generally parallel with the bottom wall of the housing atits outer periphery, but curves upwardly toward the intake opening atits inner periphery.

The circulator moves the liquid downwardly through the centralreservoir, then outwardly to the outer periphery of the lower region ofthe housing 13, and then upwardly through the tubular passages 24 whichare arranged in annular array around the outside of the funnelshapedwall 18. As the sea water flows upwardly through the vertical tubes 24,it is brought into out-of-contact heat-exchange relationship with afluid at higher temperature, thereby raising the temperature of the seawater above the boiling point established by the pressure within theupper vapor region of the vaporization chamher. The velocity ismaintained high enough to prevent the vapor from being evolved withinthe vertical heat exchange tubes. As soon as the liquid emerges from theupper ends of the heat exchange tubes, a substantial amount of theliquid flashes into vapor. The unvaporized sea water flows across theupper heat exchange surface 25 and returns to the upper end of thecentral reservoir to complete the cycle of flow described above.

The vapor evolved from the liquid flows upwardly and thence outwardlythrough a moisture-separating wall 26 accommodated within the uppervapor region of the vaporization chamber. This flow of vapor isfacilitated by a compressor-blower 27 accommodated within the housing13, but outside the separator wall 26. The compressor-blower is drivenby an electric motor (not shown) located outside of the housing 13through a shaft 28.

The separator wall 26 may take various forms, such J3 as a perforatedwall having bafiies 26a interposed in the passages thereof. Moistureformed on the inside surface of the wall 26 falls back into the lowerliquid region of the vaporization chamber. Moisture formed on theoutside surface of the separator wall flows downwardly and thenoutwardly along the sloped surface of the base of the wall 26 andreturns to the vaporization chamber through a drain pipe 29.

The compressed vapor flows downwardly through a duct 30 to a vapor chest31 which accommodates the heat exchange tubes 24. The compressed vaporis condensed while in out-of-contact heat-exchange relationship with thesea water within the tubes 24, thereby transferring its latent heat tothe liquid to be distilled. The higher the compression of the vapor, thegreater is the amount of heat transferred to the liquid to be distilled.It is possible to carry on the distillation operation once the operationis initiated by only the energy expended in driving thecompressor-blower 27.

The distillate is withdrawn from the vapor chest 31 through a dischargeconduit 32 and conducted thereby to a tank 33. The tank 33 holds aquantity of distillate which is useful particularly during starting ofthe apparatus. Vapor evolved in the tank 33 during operation flows backinto the vapor chest through the conduit 34.

The distillate accumulated within the tank 33 is Withdrawn therefrom bya pump 35 through a conduit 36, a branch conduit 37, a float-controlledvalve 38 within the tank 33, a flow switch 39, a one-way valve 40 andthe feed heater 14 before being conducted to a storage tank. Theremainder of the distillate discharged by the pump 35 flows through aconduit 37a as will be described below in more detail. Thefloat-controlled valve 38 operates to maintain a certain level ofdistillate within the tank 33. As the level of the liquid falls belowthat level, the float-controlled valve closes, decreasing or stoppingentirely the flow through the conduit 37. When the level of the liquidwithin the tank is high enough, the float-controlled valve opens.

The concentrated liquid is drawn off from the bottom of the vaporizationchamber through a conduit 41 by a blowdown pump 42 and then through aone-way valve 43 and the sea water preheater 14 wherein it is broughtinto heat-exchange relationship with the sea water introduced into thevaporization chamber.

A vent conduit 45 having inlets 45a, 45b and 450 at different levels isprovided to remove the air and any other non-condensible gases from thevapor chest 31. This air mixed with steam is 100% saturated.Consequently, a certain amount of steam is removed with the air.However, the steam is in large part recovered within the condenser 46and returned to the tank 33 via a conduit 47.

The non-condcnsible gases are removed from the condenser 46 through aconduit 48 by a water-cooled vacuum pump 49. The cooling Water for thecondenser is supplied from the intake conduit 11 through conduits 12aand 50. This coolant is discharged as waste of the condenser via aconduit 51. The supply of coolant to the condenser is controlled by asolenoid-actuated on-olf valve 52, interposed in the conduit 12a and anautomatic solenoid-actuated valve 53 controlled by a thermostat 54within the vaporization chamber. The valve 52 is opened as soon as themotor which drives the pump is started. The valve 53 controls the flowof coolant to the condenser 46 through the conduit 50 and the flow ofcoolant to the pump 49 through the conduit 55 in a manner to bedescribed.

The auxiliary heat required by the system while it is in operation orfor starting is supplied by electric immersion heaters 60 accommodatedwithin a tank 61 adjacent the duct 30. Distillate is supplied to thetank 61 by the pump 35 through the conduits 37a and 63. The vaporevolved within the tank 61 by the heat supplied by the immersion heatersis introduced into the duct 30 through a connecting port 64. The heatersare automatically controlled by the thermostat 54 so that they willsupply the necessary heat for initiating the operation of the apparatusor during the operation of the apparatus as additional heat is required.

Sealing water for the shaft 28 is supplied to the intake of thecompressor-blower 27 through a conduit 66 which communicates with thedischarge side of the distillate pump through the conduit 37a. Asolenoid-controlled valve 67 interposed in the conduit 66 is openedunder the control of the fiow switch 39 when the apparatus isfunctioning to manufacture distillate in a sufiicient quantity.

In starting up the apparatus, the reservoir 18 is filled and themanually controlled valve 15 in the conduit 12 is opened. However, theautomatically controlled valve 16 is closed preventing the supply of seawater to the reservoir. Also, the valve 67 in the conduit 66 is closedduring the starting period. The immersion heaters 60 heat the liquidwithin the tank 61, supplying vapor to the vapor chest 31. As the vaporis condensed within the vapor chest, it gives up heat to the liquidwithin the tubes 24 to evolve vapor from the liquid Within the liquidregion of the vaporization chamber. This vapor rises through thevaporization chamber, is compressed by the compressor-blower 27 and isintroduced into the vapor chest 31 through the duct 39. The liquidwithin the tank 61 is replenished from the tank 33 during this initialoperation. This, in turn, reduces the level of liquid within the tank 33so that the valve 38 close the conduct 37, causing the pump 35 to feedthe water through the conduit 37a to the tank 61. As the manufacture ofdistillate increases and the level of the liquid Within the tank 33rises, the float-controlled valve 38 will open, permitting distillationto flow through the conduit 37. When this fiow increases to aboutone-third of the units rated capacity, the flow switch 39 is actuated,opening the solenoid-controlled valves 16 and 67, the former admittingadditional sea Water to the vaporization chamber and the lattersupplying sealing water to the shaft 28.

When equilibrium is established and the apparatus is operated at ratedcapacity, the present process requires that a relatively low pressure bemaintained within the vapor region of the vaporization chamber, therebyestablishing a low boiling point within the range of from F. to F. Withproper control of the pressure within the vaporization chamber, of thevelocity of the continuously circulated sea water within thevaporization chamber and of the concentration of the sea water, sealingwithin the tubes 24 can be eliminated or virtually so when distillingsea water.

The rotary circulator 19, as explained above, maintains a continuoustoroidal circulation pattern within the liquid region of thevaporization chamber. This circulation pattern affords a relativelyshort hydraulic circuit and keeps at a minimum the expenditure ofhorsepower to maintain the continuous flow. Furthermore, part of thehydraulic circuit itself, that is, the bottom wall of the housing 13 andthe spaced-apart wall 23 serve as the housing for the circulator,thereby simplifying the design of the evaporator and effecting savingsboth in space and material.

In flowing from the liquid reservoir to the heat exchange tubes and backto the liquid reservoir, the liquid travels in a closed path, that is tosay, through a full 360 degree circuit. Since the pumping friction fornonlinear flow is greater than for linear flow and the dissipation ofenergy increases with the total number of degrees through which theliquid must turn, the design of the rotary circulator 19 and the flowpath is especially important. More specifically, the circulator itselfimparts a 90 or one-quarter turn to the liquid while at the same timeproviding the shortest possible flow path for the liquid. In addition,in making the bottom of the vaporization chamber a modified centrifugalpump volute, the total length of the hydraulic path is held to aminimum, thereby contributing to the efficiency of the circulatorysystem, an advantage over and above the compactness, low cost andsimplicity which are inherent in the design.

To maintain a constant temperature and pressure within the vaporizationchamber, it is necessary to compensate for variations in the temperatureof the sea water introduced into the vaporization chamber. When coldersea water is introduced, it is necessary to supply auxiliary heat by theimmersion heaters 60 which operate under the control of the thermostat54. When warmer sea water is introduced, there is a problem of disposingof excess heat even though the immersion heaters are inoperative. Thisdisposal of the heat in the system under consideration is accomplishedby increasing the rate of withdrawal of vapor from the vapor chest 31through the conduit 45, by increasing the quantity of coolant suppliedto the vacuum pump 49 or by increasing the flow of coolant through thevent condenser. As will be described below, in this system, it isaccomplished by increasing the flow of coolant through the vacuum pumpuntil it is at maximum and then by increasing the flow of coolant to thevent condenser.

Essentially, the purpose of the vacuum pump 49 is to remove air andnon-condensible gases, and for this purpose any suitable pump can beused. The vacuum pump is preferably a water cooled ring type which iscooled by sea water supplied to the pump via the conduit 55. Acharacteristic of this type of pump is that at a constant low pressurebelow atmospheric pressure, it has a high air-handling capacity at lowcooling Water temperatures; conversely, with increasing coolanttemperatures the air-handling capacity of the pump decreases to a pointin which its operation at the constant low pressure becomes somewhaterratic. This is the result of the temperature of the coolantapproaching or reaching the boiling point established by the lowpressure. When the coolant reaches the temperature of vaporization (thatis, at the low established pressure) the pump 49 can no longer pump air,inasmuch as steam fills the pumping spaces. Therefore, the ventcondenser 46 is used to protect the vacuum pump from excess steamreaching it after the pump was already supplied with a maximum supply ofcoolant. The condensate manufactured in the vent condenser, of course,is salvaged and as explained above, delivered to the tank 33.

The withdrawal of increased vapor from the vapor chest 31 to removeexcess heat in the manner described above has the advantage of providinga high rate of flow therethrough, thereby affording the best possibleflushing out of air and non-condensible gases. This not only helpsmaintain constant temperature and pressure within the vaporizationchamber, but contributes to the efiiciency of the apparatus.

Control of the high vacuum (low pressure) vapor compression distillationsystem of the present invention is achieved by the regulation of thecoolant supplied to the vacuum pump 49 and the coolant supplied to thevent condenser 46 by the control valve 53. At low sea watertemperatures, the water educted from the vapor chest 31 is held at aminimum required by reducing or cutting off the flow of coolant to thevent condenser via the conduit 50 and by throttling the flow of coolantto the vacuum pump 49 only to the minimum necessary to provide thenecessary water seal for the pump. At higher feed water temperatures, inorder to maintain constant evaporating temperature and pressure, it isnecessary to dissipate some of the usable heat. Initially this isaccomplished by increasing the flow of the coolant to the pump 49 underthe control of the temperature-responsive valve 53. However, this can beincreased only until the maximum Water allowable through the pump isreached, i.e., a point is reached at which further increase of coolantto the pump 49 results in reduced pump capacity and in increasedhorsepower consumption. The control valve 53 also limits the flow to thepump 49 to the maximum allowable. At still higher feed watertemperatures, there is no further increase in the supply of coolant tothe pump (i.e., once the maximum rate of flow has been established), butthe control valve 53 begins to increase the flow of coolant to thecondenser more rapidly to dissipate heat.

It is apparent that this control system provides automatic regulation ofthe coolant supplied to both the pump 49 and the condenser 4-6 under thecontrol of the temperature-responsive control valve 53 in order tomaintain heat balance conditions throughout the full range of feed watertemperature encountered, and at the same time provides means foreliminating air and non-condensible gases from the system.

The solenoid-actuated valve 52 interposed in the conduit 12a remainsopen while the motor which drives the pump 10 is in operation. Insmaller apparatus, it is feasible to drive the pumps it 35 and 42 fromthe same motor.

The distillation unit, as thus far described, is suitable for carryingout the process which is about to be described for eliminating scalingon the surfaces of the heat exchange tubes 24 with which the sea watercomes into contact. It should be understood that the process hereindescribed and claimed is not necessarily employed in conjunction withthis particular distillation unit.

The term scaling or precipitation, as used herein, refers to thedeposits which tend to form and build up on the heat exchange surfacesof sea water vaporizing apparatus. Although the exact composition of thescale will vary somewhat from location to location, the scale is for themost part magnesium hydroxide with lesser amounts of calcium carbonateand calcium sulphate.

In sea water distillation apparatus, the formation of scale causesgradual reduction in the eiiiciency and rate of output of distillate. Inthermocompression apparatus, the most eflicient apparatus for distillingsea water, scale formation not only decreases the output of the unit,but increases the horsepower necessary to drive the vapor compressor,that is, the compressor 27 in the apparatus shown in FIGURE 1. Theinefliciency caused by sealing greatly increases the cost ofmanufacturing distillate; therefore, it is customary to shut down theunits every three or four weeks to remove the scale. Even if efficiencyis not the most important factor in a particular installation, frequentremoval of scale is dictated because thick scale is more difficult toremove than thin scale, and besides, there is danger that thickparticles of scale will flake off during operation and foul up, damageor cause undue wear to the blowdown pump. Conventional methods ofremoving scale include acid and chemical treatment, ultrasonicvibrations, etc. They are time-consuming, costly and, most serious ofall, require shutdown and restarting of the apparatus.

It is well known that there are many factors which will influence thetendency and rate of scaling of a heat exchange surface by sea waterduring the vaporization thereof. These factors include the retentiontime of the sea water within the vaporization chamber, the quantity offoreign matter in the sea water, the roughness of the heat exchangesurface, the type of metal with which the sea water comes into contactwithin the distillation unit, and the carbon dioxide equilibrium of thesea Water. These factors however, are important only if scalingconditions are permitted to exist in the vaporization chamber. That isto say, they will influence the tendency and rate of scaling in somedegree once scale begins to form, but are of no significance whatever ifscaling is prevented by proper control of the more critical factors ofthe process of the present invention.

The present invention results from a discovery that scaling of thesurfaces of a vaporization chamber of a sea water evaporation ordistillation apparatus can be eliminated by proper control of certainoperational conditions.

One of the primary factors influencing the build-up of scale is thetemperature of the heat exchange surface, that is, the surface of theheat exchange tube 24 (see FIG- URE 2) adjacent the film or layer of seawater which is in contact therewith. Although high efficiency would seemto dictate that vapor be evolved from the sea water at a fairly hightemperature, it has been discovered that heat exchange surfaces tend toscale more rapidly from a condition entirely free of scale when thetemperature of the heat exchange surfaces exceeds about 120 F. (otheroperating conditions being suitably controlled in accordance with thisinvention). On the other hand, it is not feasible to maintain thetemperature of the heat exchange surfaces less than about 95 F. duringoperation. For all practical purposes, an optimum temperature range forthe heat exchange surfaces of the tubes 24 is between 105 F. and 115 F.Obviously, in order to maintain this temperature range, it is essentialto maintain the temperature of the compressed vapor within the vaporchest 31 within this range.

The impurity of the sea water (i.c., total solids dissolved therein)which is to be distilled, has a bearing on the maximum temperature whichcan be maintained while still affording the improved and unexpectedresults of the present invention. For example, if sea water having anexceptionally low impurity content is being distilled, it may bepossible to increase the temperature of the compressed vapor within thevapor chest 31 up to about 120 F. with out causing appreciable scalingof the heat exchange and other surfaces of the distillation unit.

The curve shown in FIGURE 3 represents the relationship between scalingand temperature of the heat exchange surface for sea water from thecoastal regions of the Gulf of Mexico, or in other words, sea water ofrelatively high impurity content (normal sea water"). It is based ondata obtained utilizing various atmospheric thermocompression typedistillation apparatus operated under conditions which include (a) amean temperature differential of from 6 to 9 F. between the temperatureof the water in the heat exchange tubes 24 and the temperature of thecompressed steam, and (b) an average ratio of approxi mately 1.5 betweenthe rate of feed of sea water to the unit and the rate at whichconcentrated sea water is removed. The purpose of FIGURE 3 is not toprovide operating data for a specific apparatus, but to establish thegeneral proposition that there is in fact a temperature of the heatingsurface at which scale-free operation is possible at relatively lowconcentrations.

In order to make possible the vaporization of the sea water at lowtemperatures of this order, it is essential that a high vacuum (lowabsolute pressure) be maintained within the vapor region of thevaporization chamber. Since vaporization of the sea water is usuallycarried on with a mean temperature differential of between F. and 12 F.between the temperature of the heat exchange surfaces and thetemperature of the sea water to be vaporized, this temperaturedifferential should be considered in establishing the pressure of thevapor region of the vaporization chamber.

The tendency to scale and the rate of scaling of the heat exchangesurfaces is appreciably influenced by the concentration of sea waterwithin the liquid region of the vaporization chamber. More specifically,a relatively high rate of discharge of concentrated sea water isrequired in order to operate under scale-free conditions. Theconcentration of the sea water within the liquid region of thevaporization chamber can be conveniently expressed in terms of theconcentration ratio, that is to say, the ratio between the rate of feedof sea water to the liquid region of the vaporization chamber and therate of removal of concentrated sea water therefrom. By way ofillustration, if the rate of feed of the sea water to the liquid regionof the vaporization chamber is one gallon per minute, the rate ofmanufacture of distillate is .25 gallon per minute, and the rate ofremoval of concentrated sea water therefrom is .75 gallon per minute,the concentration ratio is 1.33 (1 gal. per min/.75 gal. per min.).

FIGURE 4 is a curve showing the concentration ratio plotted againsttemperature of the heating surface with one side of the curverepresenting the conditions under which precipitation or scaling willform and the other side representing the conditions under which noprecipitation or scaling will form. The curve of FIGURE 4 is based onexperiments carried out with sea water from the coastal region ofPensacola, Florida. This curve shows that scaling can be avoided at arelatively low operating temperature of the heat exchange surface ofabout F. with a concentration ratio up to about 1.7, and at a relativelyhigh temperature of F. (i.c., relatively high for purposes of thisprocess) with a concentration ratio up to about 1.3. As for the lowlimit of concentration ratio, it is impractical and inefficient tooperate at a concentration ratio of less than 1.2, since to do sorequires too large and costly a feed heater, and a greater expenditureof energy in pumping the concentrated sea water through the dischargeconduit 40 in relation to the amount of distillate manufactured. For allpractical purposes, therefore, a concentration ratio in the range offrom 1.2 to 1.5 is necessary for scale-free operation, and aconcentration ratio of between 1.3 and 1.4 is regarded as optimum.

Although the temperature of the heat exchange surfaces and theconcentration ratio of the sea water in the liquid region of thevaporization chamber are related primary factors which must becontrolled to eliminate the formation of scale on the heat exchangesurfaces, nevertheless, another important factor is the forcedcirculation of the sea water by the rotary circulator 19 to preventlocalized concentration. The function of the rotary circulator is toimpart velocity and resulting turbulent flow to the sea water within thetubes 24 and also to build up pressure within the tubes 24 to preventthe vaporization of the sea water in the major part of the tube.Turbulent flow causes rapid continuous mixing, and near the tube exitssome minor flashing and vaporization takes place; the turbulent flowprevents localized concentration near the tube exits. The pressuresubstantially prevents the liquid from vaporizing .until it isdischarged from the upper ends of the heat exchange tubes 24, whereuponsome of the sea water flashes into vapor (less than 1% of the amountcirculated). Although vaporization of the liquid within the tubes 24 isto be avoided, it is not too serious if some small steam bubbles areformed near the upper ends of the heat exchange tubes due to the drop ofpressure, since the velocity of the water will sweep them away andprevent localized concentration.

Other advantages of imparting a relatively high velocity to the seawater within the heat exchange tubes 24 are that the liquid has atendency to scour away scum and other foreign matter from the heatexchange surfaces, and the heat transfer rate is materially improved.Nevertheless, these are only an incidental advantage of the rotarycirculator and not the paramount purpose thereof.

Although the velocity of the liquid is meaningful only in relation to aparticular apparatus, in the apparatus described above in connectionwith FIGURE 1, the desired objects can be achieved by imparting avelocity of between 5 and 6 feet per second to the sea water through thetubes 24. In most apparatus, a velocity in the range of from 2 to 10feet per second will make possible the elimination of localizedconcentration within the heat exchange passages.

By way of illustrating the process of the present invention, a specificexample will be described with reference to the apparatus of FIGURE 1,bearing in mind, however, that the process is not limited to the use ofthis particular apparatus. The liquid region of the apparatusaccommodates 14 gallons, and the unit manufactures 500 gallons ofdistillate per day. A vacuum of approximately 27.61 in. Hg (absolutepressure 2.31 in. Hg) is maintained in the vaporization chamber. Thefeed water is strained, preheated and introduced at 1.46 gallons perminute into the vaporization chamber at a temperature of approximately104 F. The circulator circulates the sea Water through bronze heatexchange tubes 24 at a velocity of approximately 6 feet per second. Thecompressed vapor is introduced through the duct 36 into the vapor chest31 at a temperature of approximately 112 F. The tubes 24 of the heatexchanger through which the feed water is circulated are .25 in. indiameter and are 32 in. in length. There are a total of 434 such tubes.The mean temperature of the circulating sea feed water within the liquidregion of the vaporization chamber is 108 F. The concentrated solutionis withdrawn from the vaporization chamber at a rate of 1.10 gallons perminute, establishing a concentration ratio of 1.33.

When normal sea Water is distilled according to the process outlinedabove, the rate of crystallization and formation of scale on the heatexchange surfaces is completely eliminated. Any scale that might form indistilling waters which have a high tendency to crystallize, precipitateand scale would be at a rate less than A to the rate of crystallizationin forming of scale under the best controlled conditions heretoforeknown. Thus, whereas most conventional apparatus requires treatment toeliminate scale and other debris every two weeks, the process of thepresent invention will probably not require cleaning when used withnormal sea water feed, or more than once every six months with unusuallyconcentrated sea water.

In order to prevent any misunderstanding, it should be pointed out thatthe composition of sea water varies throughout the world and, therefore,the optimum control conditions for scale-free operation will varysomewhat from location to location. To make matters worse, except forshipboard and drilling rig evaporators, most sea water distillationapparatus are operated in the coastal regions, or that is to say, watersin which there is the most variation. Nevertheless, although the optimumoperating conditions will vary somewhat, it is believed that they willfall within the outside ranges specified herein.

The question may arise as to why this process for eliminating scale indistillation apparatus has not heretofore been recognized by personsskilled in the art. One possible explanation is that prior to thisinvention there were many misconceptions regarding the factors which areof primary importance in the elimination of scale in the distillation ofsea water. One important and highly publicized study resulted in theerroneous conclusion that scale formation was actually greater at lowtemperatures, and as of the time of this invention this was a view heldby many persons skilled in the art. This erroneous conclusion wasapparently based on a study which placed undue emphasis on scalethickness rather than the actual quantity of scale build-up in terms ofweight per unit area.

Sea water has been known to have a reverse solubility curve, that is tosay, the chemicals dissolved in the water are less soluble at highertemperatures and precipitate out at some specific temperature. It wasdiscovered by the inventor that it is not the average temperature of themass of sea water flowing through the heat exchange tubes (that is, thetemperature which normally would be observed by reading a thermometerplaced therein) which is important in causing precipitation, butinstead, the temperature of the film of sea water immediately adjacentthe heat exchange surface, or, stated another way, the heat exchangesurface itself. Other important steps were the appreciation of thecriticality of the concentration ratio, and the importance of preventing1ocalized concentration. The present process, therefore, remainedunavailable to the art until this invention.

The invention has been shown in preferred form and by way of exampleonly, and obviously many variations and modifications may be madetherein without departing from the spirit of the invention. Theinvention, therefore, is not limited to any specified form or embodimentexcept insofar as such limitations are set forth in the appended claims.

I claim:

1. A process for evolving vapor from sea water within a vaporizationchamber to prevent formation of scale on the heat exchange surfacesthereof comprising the steps of feeding sea water to the lower liquidregion of the vaporization chamber, the liquid region of thevaporization chamber containing a liquid downtake passage and aplurality of heat exchange surfaces defining flow pas sages, regulatingthe temperature of the heat exchange surfaces to be within the range offrom F. to 120 F., maintaining a pressure within the vapor region of thevaporization chamber sufficiently low to evolve vapor from the liquidwithin the liquid region, circulating the sea water from the liquiddowntake passage through the flow passages and back to the downtakepassage at a velocity and pressure sufiicient to prevent vapor frombeing evolved in the flow passages, and removing concentrated sea waterfrom the liquid region of the vaporization chamber so as to maintain aconcentration ratio within the range of from 1.2 to 1.5 but notexceeding the concentration ratio represented by the curve of FIG- URE 4for the temperature at which the heat exchange surfaces are maintained.

2. A process as set forth in claim 1 in which the temperature of theheat exchange surfaces is regulated to be Within the range of from F. toF.

3. A process for distilling sea water which eliminates the formation ofscale on the heat exchange surfaces which define liquid flow passages inthe lower liquid region of a vaporization chamber comprising the stepsof introducing sea water into the liquid region of the vaporizationchamber, regulating the temperature of the heat exchange surfaces to avalue not to exceed F., maintaining a pressure within the vapor regionof the vaporization chamber low enough to evolve vapor from the liquidin the liquid region thereof, removing sea Water from the liquid regionso as to maintain a concentration ratio of less than 1.7 and notexceeding the value represented by the curve of FIGURE 4 for thetemperature: at which the heat exchange surfaces are maintained, andcirculating the sea water within the liquid region through the flowpassages at a velocity and pressure suflicient to prevent vapor frombeing evolved in the flow passages.

4. A process for distilling sea water as set forth in claim 3 in whichthe temperature of the heat exchange surfaces is regulated so as not tofall below 95 F.

5. A process for distilling sea water as set forth in claim 3 in whichthe concentration ratio is maintained no lower than 1.2.

6. A process for distilling sea water which eliminates the formation ofscale on the heat exchange surfaces comprising the steps of introducingsea water into a vaporization chamber which includes a lower liquidregion and an upper vapor region, regulating the temperature of the heatexchange surfaces to be within a range of from 95 F. to 120 F.,maintaining a pressure within the upper vapor region of the vaporizationchamber low enough to evolve vapor from the sea water within the liquidregion, circulating the liquid within the liquid region of thevaporization chamber at a velocity and pressure suificient to preventvapor from being evolved in the flow passages, and maintaining aconcentration ratio within the range of from 1.2 to 1.5 within theliquid region of the vaporization chamber but not exceeding theconcentration ratio represented by the curve of FIGURE 4 for thetemperature at which the heat exchange surfaces are maintained.

7. A process as set forth in claim 6 in which the tem- 11 perature ofthe heat exchange surfaces is regulated to be within the range of from105 F. to 115 F.

8. A process for evaporating sea water which eliminates the formation ofscale on the heat exchange surfaces of the vaporization apparatuscomprising the steps of feeding the sea water to be evaporated to thelower liquid region of a vaporization chamber, the lower liquid regionof the vaporization chamber containing means defining a liquid downtakepassage and a plurality of heat exchange surfaces defining flowpassages, regulating the temperature of the heat exchange surfaces to bewithin the range of from 105 F. to 115 F., regulating the pressurewithin the upper vapor region of the vaporization chamber to a valuewhich permits vapor to be evolved from the sea water within the liquidregion, circulating the liquid from the downtake passage through the Howpassages and back to the downtake passage at a velocity and pressuresufficient to prevent vapor from being evolved in the flow passages,removing the vapor evolved within the upper vapor region of thevaporization chamber, and removing sea water from the liquid region ofthe vaporization chamber at a rate sufficient to maintain aconcentration ratio of less than 1.5 within the liquid region of thevaporization chamber and not exceeding the concentration ratiorepresented by the curve of FIGURE 4 for the temperature at which theheat exchange surfaces are maintained.

9. A process for distilling sea water by thermocompression whicheliminates the formation of scale on the heat exchange surfaces of thethermocompression distillation apparatus comprising the steps of feedingthe sea water to be distilled to the lower liquid region of thevaporization chamber, the lower liquid region of the vaporizationchamber containing means defining a liquid downtake passage and aplurality of heat exchange surfaces defining flow passages, compressingthe vapor evolved in the upper vapor region of the vaporization chamber,condensing the compressed vapor in heat-exchange relationship with theheat exchange surfaces, whereby the latent heat of compression istransferred to the sea water to be vaporized within the lower liquidregion of the vaporization chamber, regulating the temperature of theheat exchange surfaces to be within the range of from 105 F. to 115 F.collecting the condensed vapor, regulating the pressure within the uppervapor region of the vaporization chamber to a value which permits vaporto be evolved from the sea water within the liquid region, circulatingthe liquid from the downtake passage through the flow passages and backto the downtake passage at a velocity and pressure sufficient to preventvapor from being evolved in the flow passages, and removing sea waterfrom the liquid region of the vaporization chamber at a rate sufficientto maintain a concentration ratio within the range of from 1.2 to 1.5within the liquid region of the vaporization chamber but not exceedingthe concentration ratio represented by the curve of FIGURE 4 for thetemperature at which the heat exchange surfaces are maintained.

10. A process for evolving vapor from sea water within a vaporizationchamber to prevent formation of scale on the heat exchange surfacesthereof, said liquid region of the vaporization chamber containing adowntake passage, a plurality of heat exchange surfaces, and acirculator for maintaining flow of the liquid from the downtake passagepast the heat exchange surfaces back to the downtake passage, saidprocess comprising the steps of feeding sea water to the vaporizationchamber, preheating the sea water to a temperature not to exceed 120 F.,circulating the sea Water from the downtake passage past theheatexchange surfaces and back to the downtake passage at a velocity andpressure sufficient to prevent vapor from being evolved on the heatexchange surfaces, removing vapor from the vapor region of thevaporization chamber, compressing the vapor and then condensing thecompressed vapor in heat-exchange relation with the heat exchangesurfaces, the exchange of heat being regulated so as not to increase thetemperature of the heat exchange surfaces above 120 F., and removing seawater from the vaporization chamber at a rate so as to maintain aconcentration ratio within the vaporization chamber which does notexceed 1.5 and also not exceeding the concentration ratio represented bythe curve of FIGURE 4 for the temperature at which the heat exchangesurfaces are maintained.

11. A process for evolving vapor from sea water within a vaporizationchamber to prevent formation of scale on the surfaces of the heatexchange passages and to minimize corrosion thereof in which thevaporization chamber includes an upper vapor region and a lower liquidregion, the lower liquid region containing a downtake passage, aplurality of heat exchange passages surrounding the downtake passage,and an axial-intake centrifugaldischarge circulator below the downtakepassage which deflects the liquid downwardly and outwardly, said processcomprising the steps of introducing sea water to the vaporizationchamber at a temperature not to exceed 120 F., circulating the sea waterby means of said circulator downwardly through the downtake passage andthen downwardly and outwardly through the circulator and then upwardlythrough the heat exchange passage surfaces and finally back to thedowntake passage at a velocity and pressure sufficient to prevent vaporfrom being evolved in the heat exchange passages, said circula tionbeing confined to the liquid region of the vaporization chamber,maintaining a low vacuum pressure within the vapor region of thevaporization chamber to fiash part of the sea water into vapor as itemerges from the upper ends of the heat exchange passages, heating theheat exchange surfaces to a temperature in the range of from F. to F.,and removing sea water from the vaporization chamber at a rate tomaintain a concentration ratio within the liquid region substantiallybelow 1.7 and not exceeding the concentration ratio represented by thecurve of FIGURE 4 for the temperature at which the heat exchangesurfaces are maintained.

12. A process for evolving vapor from sea water within a vaporizationchamber to prevent formation of scale on the surfaces of the heatexchange passages and to minimize corrosion thereof in which thevaporization chamber includes an upper vapor region and a lower liquidregion, and the liquid region contains a funnel-shaped downtake passage,a plurality of heat exchange passages surrounding the downtake passageand an axial-intake, a centrifugal-discharge circulator below thedowntake passage which deflects the liquid downwardly and outwardly,said process comprising the steps of introducing sea water to thevaporization chamber at a temperature not to exceed 120 F., acceleratingthe sea water by means of said circulator downwardly through thefunnel-shaped downtake passage and then circulating it downwardly andoutwardly through the circulator and then upwardly through the heatexchange passages and finally back to the downtake passage at a velocityand pressure sufiicient to prevent vapor from being evolved in the heatexchange passages, maintaining a low vacuum pressure within the vaporregion of the vaporization chamber to flash part of the sea water intovapor as it emerges from the upper ends of the heat exchange passages,removing vapor from the vapor region of the vaporization chamber,compressing the vapor and then condensing the compressed vapor inheat-exchange relation with the liquid in the heat exchange passages,withdrawing condensate and gases from the vaporization chamber, saidcompression and withdrawal of condensate and gases being regulated tomaintain the temperature of the heat exchange surfaces in the range offrom 95 F. to 120 F., and removing sea water from the vaporizationchamber at a rate so as to maintain a concentration ratio within theliquid region in the range of from 1.2 to less than 1.7 but notexceeding the concentration ratio represented by the curve of FIGURE .1314 4 for the temperature at which the heat exchange surfaces OTHERREFERENCES are maintained- Ellis, 0. 13.: Fresh Water from the Ocean,N.Y.,

13. A process as set forth in claim 12 including the R 1dP C 1954 15145Step of isolating the liquid P g through the downtake Saline WaterConversion, US. Dept. of Interior, passage from the compressed vapor. 5Washington, DC, January 1956, pp, 12, 13.

Symposium on Saline Water Conversion, pp. 44-50, References Cited by theExaminer National Academy of Sciences-National Research Coun- UNITEDSTATES PATENTS cil, Washmgton, DC, 1958, publication 568,

2 19 453 11 1952 Andersen 202.45 10 NORMAN YUDKOFF, Primary Examiner.2,863,501 12/1958 Farnsworth 202-75 X M. H. SILVERSTEIN, AssistantExaminer.

1. A PROCESS FOR EVOLVING VAPOR FROM SEA WATER WITHIN A VAPORIZATIONCHAMBER TO PREVENT FORMATION OF SCALE ON THE HEAT EXCHANGE SURFACESTHEREOF COMPRISING THE STEPS OF FEEDING SEA WATER TO THE LOWER LIQUIDREGION OF THE VAPORIZATION CHAMBER, THE LIQUID REGION OF THEVAPORIZATION CHAMBER CONTAINING A LIQUID DOWNTAKE PASSAGE AND APLURALITY OF HEAT EXCHANGE SURFACES DEFINING FLOW PASSAGES, REGULATINGTHE TEMPERATURE OF THE HEAT EXCHANGE SURFACES TO BE WITHIN THE RANGE OFFROM 95*F. TO 120* F., MAINTAINING A PRESSURE WITHIN THE VAPOR REGION OFTHE VAPORIZATION CHAMBER SUFFICIENTLY LOW TO EVOLVE VAPOR FROM THELIQUID WITHIN THE LIQUID REGION, CIRCULATING THE SEA WATER FROM THELIQUID DOWNTAKE PASSGE THROUGH THE FLOW PASSAGES AND BACK TO THEDOWNTAKE PASSAGE AT A VELOCITY AND PRESSURE SUFFICIENT TO PREVENT VAPORFROM BEING EVOLVED IN THE FLOW PASSAGES, AND REMOVING CONCENTRATED SEAWATER FROM THE LIQUID REGION OF THE VAPORIZATION CHAMBER SO AS TOMAINTAIN A CONCENTRATION RATIO WITHIN THE RANGE OF FROM 1.2 TO 1.5 BUTNOT EXCEEDING THE CONCENTRATION RATIO REPRESENTED BY THE CURV E OFFIGURE 4 FOR THE TEMPERATURE AT WHICH THE HEAT EXCHANGE SURFACES AREMAINTAINED.